Doublet lenses may be used for various purposes when it is desirable to combine two simplex lenses. The lenses may be abutting and may be of the same or different materials. Some types of doublet lenses include an air space between the lenses and are referred to as air spaced doublets.
The use of doublet lenses requires precise alignment of the lenses, to assure that the centering of the optical axis, the tilt of the lenses, and the space between the lenses is correct within allowed tolerances. Furthermore, tight tolerances are normally required to assure that the lenses produce high quality images.
Various methods are known for aligning the lenses of the doublet lens and for securing the lenses in alignment. However, each of these methods is time consuming and expensive because of the high degree of precision and the fine adjustments of position that are required. For example, the doublet lens may be created by aligning the lenses mechanically or optically. To align them, the first lens may be mounted on a fixture on a spindle, such as a rotating air bearing spindle. A mechanical indicator may be used to detect when the outside diameter and the face of the lens run true. Alternatively, a focusing autocollimator may be focused on the center of the optical surface, which is usually a spherical surface, so that the runout of the optical surface can be observed in the eyepiece of the autocollimator. The lens can be set on the spindle using a combination of these methods. The second lens can then be set on its mounting surface, which is usually in contact with the first lens. The position of the second lens is then set to be running true using a combination of the centering methods described above. After it is set in the correct position, it is fixed in position using glue to hold the lenses together or to a common lens cell. In some cases, the lenses are glued together at the optical surface, such as if they are for visible light. The process is challenging and misalignment can occur, despite following the time consuming process.